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Fast alternative hash (was: Files with identical SHA1 breaks the repo)

From: Stefan Fuhrmann <stefan2_at_apache.org>
Date: Wed, 1 Mar 2017 13:52:27 +0100

On 01.03.2017 05:17, Greg Stein wrote:
> I really like this idea.
> And we could take a copy of APR's sha1 code, and rejigger it to
> perform *both* hashes during the same scan of the raw bytes. I would
> expect the time taken to extend by (say) 1.1X rather than a full 2X.
> The inner loop might cost a bit more, but we'd only scan the bytes
> once. Very handy, when you're talking about megabytes in a stream-y
> environment.
> (and medium-term, push this dual-sha1 computation back into APR)
> On Sun, Feb 26, 2017 at 10:08 AM, Garance A Drosehn <drosih_at_rpi.edu
> <mailto:drosih_at_rpi.edu>> wrote:
> On 24 Feb 2017, at 15:46, Stefan Sperling wrote:
> >
> > I believe we should prepare a new working format for 1.10.0
> > which addresses this problem. I don't see a good way of fixing
> > it without a format bump. The bright side of this is that it
> > gives us a good reason to get 1.10.0 ready ASAP.
> >
> > We can switch to a better hash algorithm with a WC format
> > bump.
> One of the previous messages mentioned that better hash
> algorithms are more expensive. So let me mention a tactic
> that I used many years ago, when MD5 was the best digest
> algorithm that I knew of, and I didn't trust it for the
> larger files I was working with at the time:
> Instead of going with a completely different hash algorithm,
> just double-down on the one you're using. What I did was to
> calculate one digest the standard way, and then a second one
> which summed up every-other-byte (or every 3rd byte, or ...).
> So to get a collision, not only do two files have to get the
> same digest-result for all their data, but they have to also
> get the same digest-result when exactly half the data is
> skipped over.
> (I did this a long time ago, and forget the details. What
> I may have done for performance reasons was every-other-word,
> not every-other-byte)
> My thinking was that *any* single algorithm which processes
> all the data is going to get collisions, eventually. But it
> will be much harder for someone to generate a duplicate file
> where there will also be a collision when summing up only
> half of the data.
> I'm not claiming this is great cure-all solution, but just
> that it's an alternate tactic which might be interesting.
> People could create repositories with just the one digest,
> or upgrade it to use multiple digests if they have the need.
> I found a few benchmarks which suggest that sha-256 is maybe
> twice as expensive as sha-1, so calculating two sha-1 digests
> might be a reasonable alternative.

That is also known as bit-slicing. The neat thing is that
you create N (e.g. 4) interleaved sub-streams who's
checksum can be calculated *concurrently*, e.g using
SIMD instructions. so, you end up being ~3 times *faster*
than calculating the normal checksum.

Because the interleaved streams may look quite similar
(think bitmaps), you probably want to "salt" them. A simple
rotate or XOR might do - but I'm not an expert on this.
the goal is to end up with 4 reasonably independent
streams, hence sub-hashes.

So, the full sequence would look something like this:

* Split text T into interleaved sub-streams T1,..,T4
* Salt them S1 = salt(T1, 1), ..., S4 = salt(T4, 4)
* Calculate sub-stream hashes using bit-sliced code
   D1, ..., D4 = sha1_4x(S1, ..., S4) = sha1(S1), ..., sha1(S4)
* Calculate the final checksum D = sha2(D1|...|D4)

Not only would that solve the current sha1 issue but
neatly address the fact that nowadays we can read
data faster from disk that we could checksum it.

-- Stefan^2.
Received on 2017-03-01 13:52:39 CET

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